feat(userspace): add demo ring-3 programs

user/hello.c

@@ -0,0 +1,13 @@
+#include "ulib.h"
+
+int main(void)
+{
+    print("hello: greetings from ring 3, pid=");
+    printDec(userGetpid());
+    print("\n");
+
+    userYield();
+
+    print("hello: resumed after yield, exiting cleanly\n");
+    return 7;
+}